This was Gollum's final riddle from The Hobbit:
"This thing all things devours;
Bird, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slay king, ruins town,
and beats a mountain down."
With pointed fangs I sit and wait; with piercing force I crunch out fate; grabbing victims, proclaiming might; physically joining with a single bite.
What am I?